This invention relates to a method of manufacturing low carbon pressure vessel steel having a high strength and a high toughness with high resistance to hydrogen attack and overlay disbonding, and high weldability.
Cr-Mo steels for petroleum refining reactors such as 11/4Cr-1/2Mo to 3Cr-1Mo, are required to have high strength at elevated temperatures since higher operation temperature and pressure are required for improving the refining efficiency. To improve the strength, it has been the practice to enhance the hardenability by B treatment or to incorporate carbon and alloying elements to the upper limit of the specification. It has also been proposed to improve the strength by incorporating such micro-alloying elements as V, Nb, Ti, etc. However, in spite of these methods, it is still difficult to satisfy the elevated temperature allowable stress defined in ASME Sec. VIII, Div. 1 or 2 in the case of heavy section plates, as it is necessary to use long term post weld heat treatment (PWHT). Furthermore, an increase in C content strongly decreases resistance to hydrogen attack and overlay-disbonding. For those steels to which micro-alloying elements are added, higher heating temperature in hot working, normalizing or quenching are necessary than for the ordinary steels because of low solubility of carbo-nitrides of these micro-alloying elements. But although high temperature heat treatment is effective to increase the strength, toughness decreases greatly due to coarse austenitic grain size caused by high temperature heat treatment. Thus, for pressure vessel steels for elevated temperatures, especially those used in a hydrogen environment, higher temperature heat treatment described above is not feasible from the point of safety design.